With violence increasing in eastern Ukraine despite attempts at both concessions and pressure from the interim government to placate pro-Russian separatists, President Obama is coming under pressure to step up the U.S. response.

The president has pushed for a diplomatic solution in Ukraine since expanding U.S. sanctions to senior Russian government officials, individuals and a bank in late March. At the time, the administration announced it had put in place the framework for sanctions that could hit entire sectors of the U.S. economy. But despite certainty among U.S. officials that Russia is fomenting the turmoil in the eastern part of the country, there has been no further actions to penalize Ukraine.

"We are actively evaluating what is happening in eastern Ukraine, what actions Russia has taken, what transgressions Russia has engaged in, and we are working with our partners and assessing for ourselves," White House Press Secretary Jay Carney told reporters Monday. He would not say whether Russia had crossed a line that would invite the implementation of more severe sanctions.

Russian President Vladimir Putin and Mr. Obama spoke Monday afternoon at the request of the Russians, a call that a senior administration official described as "frank and direct."

"The president made clear that the diplomatic path was open and our preferred way ahead, but that Russia's actions are neither consistent with or conducive to that," the official said.

The foreign ministers of the European Union agreed to expand asset freezes and visa bans against more Russians Monday, although it was not immediately clear whether this would spur the U.S. to move to the harsher sectoral sanctions.

Samantha Power, the U.S. ambassador to the United Nations, argued over the weekend that the first round of sanctions did work by bringing the value of the ruble to an all-time low and depreciating the Russian stock market by 20 percent. But even though she spoke after the first gun battle had occurred in eastern Ukraine, there was a distinct wait-and-see approach to the crisis.

"We've seen that the sanctions can bite. And if actions like the kind that we've seen over the last few days continue, you're going to see a ramping up of those sanctions," she said.

Treasury Secretary Jacob Lew finalized a $1 billion loan guarantee to Ukraine Monday morning in a meeting with the nation's finance minister. It is the highest-profile action the U.S. has taken to help the country in recent weeks.

But pressure is mounting on the president to do more. Experts say the situation could spiral out of control, due in part to Putin's willingness to defy international pressure.

"The situation in eastern Ukraine may be localized at the moment, but there's a great risk of escalation and violence from all sorts of actors who may not be directly under either government's control," said Andrew Weiss, the vice president for studies at the Carnegie Endowment for International Peace. "The Obama administration and its European allies have struggled to find tools that might deter Putin. Having taken the military option off the table at the very start of the crisis, it's far from clear what might make Putin think twice. So far, the threat of so-called sectoral sanctions against key part of the Russian economy, obviously, hasn't had much effect."

Sen. John McCain, R-Ariz., said the U.S. should send small arms to the Ukrainians to defend themselves, and the U.S. and Europe should impose very severe sanctions even though it could have an adverse effect on the western economies.

The rising unrest, McCain said on "Face the Nation" Sunday, is the result of U.S. failure to "enact anything really meaningful and important as a result of [Russian President] Vladimir Putin's incursion and annexation under Crimea."

"Where is the president of the United States?" McCain asked. "Shouldn't the president of the United States be speaking forcefully and strongly? And didn't the president say if they carried out further actions, there would be further sanctions? So far, we haven't heard anything."

The Washington Post's editorial board argued for more action Monday, saying that a tougher Western reaction possibly could have stopped the Russian offensive and the bloodshed, which may be used as a pretext for further action.

"It may be too late to prevent war in eastern Ukraine," the editorial read. "But the United States must quickly take the measures promised by Mr. Obama and Mr. Kerry, or lose what little credibility it retains on Ukraine." It recommended providing non-lethal supplies and small arms to Ukrainian forces

Carney said that the U.S. is focusing on diplomatic efforts at the moment, and that lethal aid is not under active consideration at the moment, but the options are under review. The U.S., European Union, Russian and Ukrainian governments are scheduled to hold talks in Geneva this week.

A former U.S. government official who has dealt with Russia and Eurasia called the meeting "the key event this week."

"Until it happens, I don't expect the administration to make any decisive moves," the official said. They predicted that if the Russians pull out of the meeting or it does not produce any meaningful results to defuse and de-escalate the situation, the administration will likely go ahead with an additional round of sanctions.